Process for rapidly controlling a process variable without overshoot using a time domain polynomial feedback controller
Inactive Publication Date: 2006-07-27
FRANCIS ROBERT H
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Benefits of technology
[0043] It may be implemented with traditional sensors and final control elements.
[0044] Further objects and advantages of this invention will become apparent from consideration of the drawings and ensuing description.
[0045]FIG. 1 is a block diagram of a control system including the polynomial controller as utilized in accordance with the method of the present invention.
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Problems solved by technology
These oscillations may be extreme enough that the process material is ruined or an unsafe situation occurs.
However, PID controllers with no-overshoot tuning parameters result in relatively slow performance.
Thus, unnecessary time is required to move the process variable to the setpoint.
The most significant shortfall of the traditional PID, when used in applications where overshoot is not allowed, is that the PID does not have a feature ensuring the final control element is set OFF (as used herein, the terms OFF and ON represent succession of the control medium whether the process variable approaches the setpoint from above or below) as the process variable reaches the setpoint.
Thus, the PID controller does not have systems to prevent or minimize overshoot.
In some cases, however, that process cannot exceed that maximum setpoint without damage occurring to the environment or to the equipment or product.
Without a method to ensure the final control element is set OFF if the process variable moves beyond the setpoint, the PID controller cannot ensure this damage does not occur.
Thus batches can fail and equipment or environmental damage can occur when the PID controller is used for these applications.
The result is the process does not operate at the optimal point, increasing production times or decreasing production yields.
Setpoint suppression / reset, while commonly utilized in applications where overshoot is not allowed, also has slow performance as the controller first reduces the final control element's percent ON to meet the intermediate setpoint.
Because of this action, the controller's precision is not the quality of the traditional PID or other controllers.
Fuzzy logic currently is not supported by most industrial controllers and requires significant computing resources to implement.
The most significant shortfall of the feed-forward controller involves the requirement that the process under control be well understood.
Often when implementing these controllers, a disturbance (an event that drives the process from the setpoint) that was not anticipated by the engineer configuring the controller attacks the process.
The disturbance can make the process unstable resulting in process or equipment failure or an unsafe condition.
However, these controllers are very complex and require advanced engineering support to deploy, maintain and modify, increasing the cost of the control system.
Because of the complexity of this type of controller, significant computing resources are required to implement these controllers.
Most contemporary industrial controllers do not have these computing resources available and those that do are quite expensive.
Thus, to date, these controllers have not been widely used in industrial applications.
Contemporary model-based controllers also require that the process under control be well understood and therefore they suffer from the same short coming as feed-forward controllers when the control practitioner overlooks a source of disturbance.
As stated above, this disturbance can make the process unstable resulting in process or equipment failure or an unsafe condition.
Thus, control practitioners hesitate in implementing these controllers on new processes.
Clearly, the cost to install model-based controllers on new processes can be prohibitive.
However, this controller's precision is reduced when that motive force varies.
Method used
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[0051]FIG. 1 is a block diagram of a standard process / system in need of control with a feedback control system. This system has: a general process or system 11, a sensor 12 to measure a process variable 13, an analog controller 14 and a final control element 17 as its general equipment.
[0052] The process variable is a parameter that is an indication of the chemical or physical state of that system. The controller is a hardware or software based device that is used to calculate corrections to differences between a setpoint and the measurement. The first operation within the controller is a means to calculate an error signal 15 that is the difference between a setpoint and the process variable. The controller operates upon the error signal 15 to calculate a control variable 16. The control variable is the position at which the final control element needs to operate in order for the process variable to reach and maintain the setpoint. The final control element may be any variable outp...
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A method for controlling a process variable as it approaches a predetermined value (setpoint) so that the setpoint is not exceeded. The method employs a time domain polynomial equation in a feedback configuration and utilizes a controller that acts as an On / Off controller until the process variable approaches setpoint. As the process variable approaches setpoint, the controller acts as a fast responding analog controller thereby “tailoring” a control variable to precisely bring the process variable to the setpoint without exceeding or overshooting the setpoint.
Description
CROSS REFERENCE TO PRIOR APPLICATION [0001] This application is a continuation in part of, and claims the benefit under 35 U.S.C. § 120 of, the filing date of U.S. patent application Ser. No. 09 / 771,799, filed Jan. 29, 2001.BACKGROUND OF THE INVENTION [0002] This invention relates generally to the field of industrial process control, and particularly to a method for rapidly controlling a measured variable of a process from an existing value to a very divergent desired value without an overshoot beyond the new value. Analog Controllers [0003] An analog controller receives a continuous analog signal input that represents a measured process value or variable (PV) from a sensor and compares this value to the desired value setpoint (SP) to produce an error signal (ES). The controller uses this error to calculate any required correction and sends a continuous analog signal output (control variable), to a final control element (any continuously variable valve, damper, pump, fan, etc.). Th...
Claims
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Login to View More IPC IPC(8): G05B13/02G05B21/02G05B5/01G05B11/42
CPCG05B5/01G05B11/42G05B13/024
InventorFRANCIS, ROBERT H.
OwnerFRANCIS ROBERT H